An articulated shaft structure has been known that allows a wrist unit at a distal end of a forearm of a robot to rotate about a longitudinal axis of the forearm (e.g., see Patent Literature 1).
In this articulated shaft structure, a ring-like output hypoid gear coupled with the wrist unit is supported by a proximal end of the forearm through bearings so as to be rotatable coaxially with the longitudinal axis of the forearm. Further, a unit including a housing member that rotatably supports the input hypoid gear and also supports a motor and gears for decelerating and transmitting rotation of the motor to the input hypoid gear is mounted on a proximal side surface of a part of the forearm. This causes the input hypoid gear to engage with the output hypoid gear.